1. Field of the Invention
This invention relates to a lift cam mechanism for an electronic parts-mounting apparatus, which lifts and lowers a mounting head which is moved about a vertical shaft to pick up and mount electronic parts.
2. Prior Art
Conventionally, an electronic parts-mounting apparatus of this kind has been proposed e.g. by Japanese Laid-Open Patent Publication (Kokai) No. 5-175695, in which an index unit, which forms a main part of a driving system of the apparatus, intermittently rotates a rotary table formed in a manner suspended from the index unit to thereby bring each mounting head arranged on the outer periphery of the rotary table to a suction station and a mounting station, while each mounting head is lifted and lowered to pick up an electronic part at the suction station by vacuum and mount the same on a circuit board at the mounting station. Each electronic part is picked up by vacuum from a pick-up point at the suction station and mounted at a mounting point at the mounting station. The pick-up point and the mounting point are different in level. Therefore, apart from being lifted and lowered directly for picking up and mounting electronic parts, the mounting head is progressively lifted and lowered by the lift cam mechanism comprised of a ribbed cylindrical cam disposed on the index unit side and cam followers fixed to the mounting head, as the mounting head is moved about a vertical shaft by intermittent rotation of the rotary table.
More specifically, a pair of upper and lower cam followers (roller followers) engage with a rib portion of the ribbed cylindrical cam such that the rib portion is interposed between the upper and lower cam followers in a manner rollable thereon, and the mounting head is suspended from a holder holding the cam followers, by way of a suspension member. The upper one of the cam followers is directly and rotatably fitted through the holder, whereas the lower one is rotatably fitted through a distal end of a swing arm pivotally or swingably mounted in the holder and at the same time urged toward the rib portion of the ribbed cylindrical cam by a coil spring interposed between and the swing arm and the holder. The coil spring ensures the reliable operation of the lift cam mechanism, and accommodates variations in thickness among individual rib portions formed on respective ribbed cylindrical cams and changes in thickness at a sloped portion of the rib portion of a particular cylindrical cam to thereby enable the cam followers to smoothly roll on the rib portion of the ribbed cylindrical cam.
The lift cam mechanism of the proposed electronic parts-mounting apparatus described above, however, suffers from the problem that a swinging stroke of the swing arm is very small and hence lubricant at a support shaft of the swing arm is liable to become short (since lubricant is difficult to spread over the whole peripheral surface of the support shaft), which often results in degraded durability of the swing arm and faulty following operation of the cam followers. Further, if there remain burrs at a portion of the swing arm pivotally fitted on the support shaft, the swing arm swings to an excessive extent, which can also cause the faulty following operation of the cam followers. As the apparatus operates at a higher speed, the faulty following operation of the cam followers comes to act on the mounting head as a shock, causing displacement or deviation in position of an electronic part picked up by the mounting head.